1. Field of the Invention
The present invention relates to a light source, and more particularly to a broadband light source for outputting Amplified Spontaneous Emission (ASE) light.
2. Description of the Related Art
Erbium-doped fiber amplifier light sources, super luminescent diodes, and reflective semiconductor optical amplifier light sources have been suggested as broadband light sources for outputting incoherent light through wide wavelength bands. Although the erbium-doped fiber amplifier light sources produce stable high-power polarization-insensitive light, the produced wavelength band is limited so that the erbium-doped fiber amplifier light sources are unsuitable for the broadband light sources. In addition, the size of the erbium-doped fiber amplifier light sources is larger than the size of semiconductor devices and it is further difficult to reduce the manufacturing cost even if the erbium-doped fiber amplifier light sources are mass-produced. Super Luminescent Diodes (SLDs), on the other hand, have a large optical bandwidth and can be manufactured at a low cost. However, SLDs have a problem in that the output power is limited. Reflective Semiconductor Optical Amplifier (RSOA) light sources output spontaneous emission light that has been amplified by a highly reflective coating layer deposited on a first terminal or end of a Semiconductor Optical Amplifier (SOA). Thus, it is possible to achieve high-power broadband light sources at a low cost with RSOAs. However, if reflectivity of a second terminal of the semiconductor optical amplifier is not extremely low, a Fabry-Perot resonator is formed between the first and second terminals so that the spectrum of output light varies based on the wavelength of the output light. This variation is referred to as a “gain ripple”. Thus, it is difficult to obtain incoherent light having a uniform spectrum.
In order to solve the gain ripple problem of the reflective semiconductor optical amplifier light sources, another broadband light source has been suggested, in which an external broadband reflector is connected to the SOA and a high reflective coating layer is not deposited on the first terminal of the semiconductor optical amplifier.
FIG. 1 is a schematic view showing a structure of a conventional reflective semiconductor optical amplifier light source 100. The conventional reflective semiconductor optical amplifier light source 100 includes a semiconductor optical amplifier (SOA) 110 and an external reflector (R) 120, which are connected to each other through an optical waveguide 130.
The semiconductor optical amplifier 110 includes a gain medium 112 and first and second anti-reflective layers 114 and 116 coated on both side ends of the gain medium 112. Incoherent amplified spontaneous emission light 140 is outputted through the first and second anti-reflective layers 114 and 116.
The external reflector 120 is optically connected to the first anti-reflective layer 114 through optical waveguide 130 and is used to reflect back the incoherent amplified spontaneous emission light 140 outputted through the first anti-reflective layer 114, such that the incoherent amplified spontaneous emission light 140 emitted is returned into the semiconductor optical amplifier 110. The distance between the semiconductor optical amplifier 110 and the external reflector 120 is preset or predetermined such that the incoherent amplified spontaneous emission light 140 returns to the semiconductor optical amplifier 110 by traveling over a length referred to as the coherence length. Coherence length is well-known in the art and need not be explained in detail herein.
The structure shown in FIG. 1 has an advantage in that a reflectivity condition needed of the first and second anti-reflective layers 114 and 116 to achieve a small gain ripple is attenuated. Thus, as a small gain ripple is generated even if reflectivity of the first and second anti-reflective layers 114 and 116 is not extremely low, an anti-reflection coating for the first and second anti-reflective layers 114 and 116 is easily achieved.
However, the above-mentioned reflective semiconductor optical amplifier light source 100 has an output power level lower than that of the erbium-doped fiber amplifier light source, so that there is a need for increasing the output power of the reflective semiconductor optical amplifier light sources and to provide a reflective semiconductor optical amplifier light source having a higher output power.